Hey Guys,

I have an 87 trans am with a zz4 crate engine and 700r4 street fighter transmission, both put in about a year ago. I’m still using the original rear end with the 2:73 gears (it’s an expensive upgrade and I figured I could hold off on it for awhile). Anyway I’ve decided its time to swap the rear end etc and am leaning towards using 3:42 or 3:50 gears. I figured this would provide a good balance without making the car scream on the highway.

Anyway quick question. This Trans Am over the past few years has become my first project car so this is my first time doing any of this. Right now the engine is rated at 355 horse power (though those are usually a little exaggerated) and said to tach out at 5900 rpms but the way the car is geared right now only allows it to go to 4500.
How much of a performance difference can I look to see by swapping out the rear end from 2:73 to 3:42/3:50?

 

No GM 3:50's that I am aware of.

JamesC

 

The available ratios for the 10 bolt are 3.23, 3.42, 3.73 and 4.10 in the range that you might be able to use. All of these require a 3 series differential, which you don't have. You have a 2 series. You can get some of these ratios with a special thick ring gear that will fit the 2 series. They cost a little more.

 

I'm going to be completely swap out the rear with a ford 9"

 

You might really like the 3.50 gear range with your setup. Sounds like a nice combo. Wish I could try it. One of my cars has 3.27 and another has 3.73 gears. Neither is perfect but close on both sides with 3.5 being about spot on center. Going with a Ford 9 give you that option. Do it and let us know how it works out for you!

If you go with a 3.5 set, PM me if you would please and give me some feedback. I'm also considering a 9 in the same ratio to replace my 9 bolt 3.27.

Regards

 

I definately will keep you posted as to how it goes. Since I've never done this before I'm really going on what I've been told (in terms of knowing what kind of performance difference this will make." How much of a difference should I see after the swap?

 

You would see more of a gain if you used a 12 bolt instead of the 9 inch Ford. The 9 inch is heavier and takes more power to turn.

 

Thanks for the input I appreciate it. once the swap is complete, how much of a difference should I expect to see in Launch concidering I'm coming from 2:73's

 

I wasn't using a ZZ4, but I've made the swap from 2.73:1 to 3.42:1, more than once,and it's noticeable. If you care more about the 1/4 mile than mileage, I'd go 3.70:1, otherwise the 3.50:1 is fine, and you'll think it's a great, fun improvement. The 3.70s would add just 120 rpm @ 65 mph.
As for the 9" eating more power, CarCraft magazine did a test several months ago, a chevelle with a 12-bolt, they also tried a 9" and a Dana 60, all on a chassis dyno. The 9" was within 5 rear wheel horse power of the 12 bolt. The extra mass will affect acceleration, but not dramatically so.
Use a good synthetic gear lube. That'll about recover those 5 horses.
If you're going 9" anyway, use at least 31 spline axles.